Conventional radio-wave beamforming applications typically use machined waveguides as feed structures, requiring expensive micro-machining and hand-tuning. Not only are these structures difficult and expensive to manufacture, they are also incompatible with integration to standard semiconductor processes. As is the case with individual conventional high-frequency antennas, beamforming arrays of such antennas are also generally difficult and expensive to manufacture. In addition, phase-shifters are required that are incompatible with a semiconductor-based design. Moreover, conventional beam-forming arrays become incompatible with digital signal processing techniques as the operating frequency is increased. For example, at the higher data rates enabled by high frequency operation, multipath fading and cross-interference becomes a serious issue. Adaptive beamforming techniques are known to combat these problems. But adaptive beamforming at 10 GHz or higher frequencies requires massively parallel utilization of A/D and D/A converters.
To address these problems, integrated circuit approaches have been developed in which the electrical feed lines and structure, active circuitry, and the antennas are all associated with a semiconductor substrate. To enhance the number of available antenna elements, a wafer scale substrate may be used such that the resulting beamforming system may be denoted as a “wafer scale antenna module.” Each antenna element in such a module may be driven with a properly-phased signal so as to transmit a signal into a desired beam-steered direction. Similarly, received signals must also be properly-phased if a particular receive direction is to be selected through beamforming. A number of “wired” driving architectures have been developed to drive the antennas. For example, each antenna (or sub-array of antennas) may be associated with an oscillator. The aggregation of an antenna (or antennas) and its oscillator may be denoted as an integrated antenna circuit. Alternatively, a centralized oscillator may be used to drive an electrically wired feed network such that the resulting signal propagating through the feed network drives the antenna elements (ignoring any phase-shifting of the propagated signal for beamforming purposes). As discussed in commonly-assigned U.S. application Ser. No. 11/141,283, a feed structure may be formed using co-planar waveguides or microstrip formed using the metal layers formed in the wafer's semiconductor manufacturing process. A synchronization signal to be transmitted is injected into an input port for the feed network whereupon the signal propagates through the feed network to the individual antenna elements. U.S. application Ser. No. 11/141,283 disclosed a distributed amplification architecture to address the substantial propagation losses introduced as the input signal propagates across the feed network.
Although the propagation losses are addressed in this fashion, a signal will also tend to degrade through dispersion as it propagates through the “wired” feed line network. Thus, commonly-assigned U.S. application Ser. No. 11/555,210 discloses an integrated antenna circuit architecture wherein each antenna (or sub-array of antennas) associates with its own oscillator. Because no signal need be driven across the wafer from a centralized oscillator to the antennas, the integrated circuit architecture advantageously has less dispersion as the signal to be propagated is generated locally and thus has relatively little dispersion introduced in the oscillator-to-antenna propagation path. An issue exists, however, in integrated antenna circuit architectures of keeping the various oscillators in synchronization. As disclosed in commonly-assigned U.S. application Ser. No. 11/555,210, a distributed amplification feed network may be modified such that the entire network resonantly oscillates in unison. The integrated antenna circuits may thus be synchronized through phase-locked loops or other techniques with regard to the globally-synchronized signal provided by the resonant feed line network. Although a resonant feed network thus provides global synchronization of the integrated antenna circuits, it is a substantial “tethered” structure to design and demands a lot of substrate space. In that regard, each integrated antenna circuit oscillator is required to be highly stable in phase and frequency with very low values of phase noise to permit accurate array phase control for beam steering. Synchronizing these oscillators through a resonant network uses valuable wafer real estate budget. In addition, the fine structure of the resonant feed is subject to attenuation, which increases with frequency, and thus increases the wafer power dissipation and eats up the wafer power budget. Moreover, there is the issue of on wafer signal propagation cross talk with other signal lines and devices and the major issue of the “near-far” effect of signal “differential” delay and latency to each of the oscillators. In addition, the un-avoidable on-wafer resonant propagation is subject to highly-frequency dependent phase distortion. These issues affect array phase control accuracy.
Accordingly, there is a need in the art for alternative synchronization, preferably “tetherless and optical” techniques for integrated-antenna-circuit-containing wafer scale antenna modules.